Electronic component assemblies that are used in “down hole” applications in well drilling logging and measurement activities are subject to significant vibration and shock present in the drill string axial direction, along both transverse axes, and rotational acceleration about the axis and high heat/thermal exposure. The electronic component assemblies survive long hours down hole only if: (1) the mounting support offers protection from acceleration induced forces that cause relative strain between components and (2) there is a sufficient heat conduit in place to transfer the heat generated by the printed circuit board (PCB) so that PCB damage does not occur. Failures result too often and have serious economic consequence.
Known mounting methods involve supporting printed circuit boards on structures called “Subs” and or chassis and suspending the assemblies in confining, pressure proof, enclosures. The pressure proof enclosures are attached to drill strings to which drill bits are attached or wire line strings where there is not drill bit attached. During a drilling operation, the drill strings are the origin of the most serious shock and linear vibration. The acceleration forces generated during drilling are transmitted by way of the drill string to the enclosure and the attached “Sub” or chassis to the electronic assembly and components. Failure of the electronic component assemblies results when: (1) the acceleration forces cause relative motion between PCBs and their attached components and or (2) high heat/thermal exposure damages the PCB since there is no conduit in place to pull the heat from the PCB and transfer it to.
Two currently used mounting options include; (1) attachment of PCBs to strongbacks with screws, with a sheet of Elastomer captured between PCBs and strongbacks and, (2) positioning PCBs in openings, or cavities, in “Subs” and filling the remaining volume with elastomer that is cast and cured in place. Although and used more frequently is when the PCB is placed in a mold (that accurately fits the geometry of the cavity in the “Sub”) cast or encapsulated, cured, removed from the mold and inserted into the cavity in the “Sub” or directly into the confining structure such as a pressure barrel. In both cases, the chassis and or “Sub,” with the electronic assemblies in place, are inserted in the bore of a confining structure, such as a tubular pressure barrel. In many cases the electronic assembly (encapsulated PCB) is placed directly into a confining structure cavity, such as a pressure barrel.
The first mounting option causes stress concentrations at the screws where acceleration forces are transmitted from the chassis and or “Sub” to the PCBs. In addition, temperature changes cause relative thermal expansion between PCBs and “Subs” or chassis and strains the screw attachment points.
The second mounting option provides cushioning for all components against acceleration forces delivered through the strongback. However, differential thermal expansion between the elastomer and “Sub” causes forced migration of elastomer in unpredictable amounts and directions. As a result, destructive strains force relative movement between PCB and attached components.
U.S. Pat. No. 4,891,734 to More et al. provides a mounting option that is based upon the premise that the ideal support for electronic components will cushion all components about equally, will allow inevitable elastomer migration, very localized, in known directions and in known amounts, and that small movements allowed by cushioning can be accommodated by free moving conductors sufficiently short and supported to prevent their becoming a vibrating independent mass.
U.S. Pat. No. 4,891,734 to More et al. discloses enclosing electronic assemblies in elastomeric bodies that are separately molded to fit the confining enclosures of strongbacks that are provided with cavities for the elastomeric bodies and configured to be received in tubular shrouds. The electronic assemblies are positioned in molds shaped to represent the cavities of the strongbacks and elastomeric material is cast around the assembly. Only a connector is exposed at the elastomer surface.
The present invention provides molded pre-forms, methods to fabricate the molded pre-forms, and their use to protect electronic components and assemblies from damage due to vibration, shock and/or thermal exposure. Particular features of the present invention relate to use of singular and co-molded pre-forms in “down hole” applications in drill and wire line strings in wells.